1. Field of the Invention
Embodiments of the present invention relate generally to polymer articles and compositions for medical devices which provide control of dimensional changes in such articles, for example, ophthalmic devices and other suitable medical and non-medical devices.
In certain embodiments, the invention relates to hybrid hard-soft contact lenses possessing improved dimensional stability, oxygen permeability, and machinability.
2. Description of the Related Art
Traditionally, the field of vision correction has involved measuring aberrations in the optics of the eye, by first creating a prescription that corrected for the measured aberrations, and then using the prescription to correct the measured aberration, e.g., by surgery, spectacles or contact lenses. Thus, the ability to correct vision aberrations has been limited by both the degree of accuracy in the measurement of the aberrations and by the ability to correct the measured aberration.
The field of vision correction is currently in the midst of a revolution. New technologies have been developed to measure a variety of aberrations in the optics of the eye to a high degree of accuracy. These new wavefront measurement techniques (such as Shack-Hartmann wavefront sensing or Talbot Interferometry) can precisely measure the eye's aberrations to such a high degree of accuracy that, at least in theory, a customized prescription could be created to correct vision so that it is better than 20/20. Recent advances in laser refractive surgery techniques, such as LASIK and photorefractive keratectomy, as well as improvements in spectacle lens manufacturing, now enable vision to be corrected using eye surgery or spectacles to a degree of accuracy that approaches the accuracy of the new measurement technologies.
However, this is generally not the case with contact lenses, particularly when the correction of higher order aberrations is desired. Popular soft contact lenses cannot currently achieve the same degree of corrective accuracy as spectacles or laser refractive surgery because of dimensional variations in the lenses resulting from conventional soft contact lens fabrication processes. Hard contact lenses, which could theoretically provide a platform to achieve the highly accurate corrections achievable by surgery and spectacles, are not as comfortable as soft contacts and generally lack positional stability on the eye.
Hybrid hard-soft contact lenses, having a relatively hard center portion and a relatively soft outer skirt portion, have been developed which could theoretically provide both a platform for a more accurate corrective prescription and also provide the comfort of soft contact lenses. In the typical process for manufacturing hybrid lenses of the rigid center/soft skirt (RC/SK) type, the rigid center polymer is pre-fabricated by polymerizing monomer to form a rigid polymer, followed by polymerization of the soft skirt components in direct contact with the preformed rigid polymer, and then hydration with water. In addition to simplifying the manufacturing process, this method of manufacture avoids the deleterious effects which the presence of the soft components may have on the properties of the rigid portion, arising from interaction of the monomers which form the hard and soft portions. Such effects may include loss of modulus, low strength, refractive index changes, and/or loss of oxygen transport properties.
A consequence of pre-fabricating the rigid portion of the lens, however, is that the rigid portion has fixed dimensions and does not easily accommodate dimensional changes that might later take place in the surrounding skirt material. For example, upon hydration of the lens, the hard and soft portions absorb water, with the soft portion swelling significantly more than the hard portion. The differential expansion between the soft portion and the hard portion results in stresses about the region of the junction between the two portions. These stresses may manifest themselves as physical distortions of the lens geometry and/or weakening of the bond between the hard and soft portions, degrading the performance of the hybrid lens.
Similarly, in other medical devices, such as cardiovascular equipment and other body implants where a hydrophilic surface is adhered to the device, stresses from hydration have been found to result in spalling of the coating and other types of failures due to unequal swelling or other distortions induced by water absorption.
The constraint arising from pre-fabrication of the rigid portion of the medical device or lens can be mitigated if the soft portion of the medical device or lens is fabricated and bonded to the rigid portion while in its fully expanded state. Such pre-expansion has been attempted using the addition of water to the skirt monomers at the time of polymerization. Unfortunately, water is not compatible with the monomers preferable in many soft lens material compositions and may cause the components or intermediates to phase separate prior to, or during, the polymerization process. Further, fully hydrated polymers are generally too soft to machine into final shape after polymerization and thus are constrained for use in fully molded products.
The incompatibility of water with the components and intermediates of the skirt portion can be avoided by replacement of water by other expansion control agents. These agents serve as temporary stand-ins for the water prior to and during polymerization and are intended to be extracted after polymerization and processing and replaced by water.
For example, U.S. Pat. No. 5,260,000 to Nandu, et al., discloses expansion control agents comprising low polarity saturated hydrocarbons and diluents of intermediate polarity. Examples of such species include alcohols, esters, ethers or acids. Low to moderate polarity agents, however, are found to result in skirts possessing poor mechanical properties when utilized in quantities high enough to allow their subsequent displacement by water. Thus, machining the skirts while the low to moderate polarity expansion control agents remain in the skirt polymer is difficult because the polymers are too soft. Furthermore, organic agents are problematic as their toxicity requires removal of leachable toxic residues from the lens, adding significant time and cost to the lens manufacturing process.
Alternative expansion control agents to simple hydrocarbons include low molecular weight, water soluble polymers. For example, U.S. Pat. No. 4,121,885 to Erickson, et al., discloses low molecular weight, water soluble oligomers such as polyoxyethylene, polyethylene glycol, polyvinyl alcohol, dextran, and polyvinylpyrrolidinone. While these oligomers reduce the toxicity issues discussed above and have lesser impact on pre-hydration mechanical properties, they suffer from low solubility in skirt monomer compositions, restricting their use to low water skirt products. Furthermore, these oligomers are difficult to extract from fabricated skirt polymers, due to their high molecular size relative to the pore size found in the skirt polymers. Additionally, the low propensity of the oligomers to crystallize leads to instability in the buttons produced with them when these fillers bloom to the surface, altering the expansion properties of the skirts so produced.
Another difficulty encountered with hybrid lenses of the prior art is the low oxygen permeability of the materials used in such lenses. Since the cornea receives its oxygen supply exclusively from contact with the atmosphere, good oxygen permeability is an important characteristic for any contact lens material. In particular, the oxygen permeability of the skirt material of prior art lenses was dependent solely on the water content of the traditional hydrogels used in these lenses. It was found that the higher the water content within the skirt material of prior art hybrid lenses, the greater was the oxygen permeability through the skirt to the cornea.
However, high water containing hydrogels have exhibited undesirable mechanical properties. These properties include poor tensile strength, which may result in tearing or other breakage, as well as an increase in the distortions about the region of the junction discussed above. The greater the water content, the greater the expansion factor to be countered by any control agent.
In order to improve and balance the oxygen permeability of the skirt in hybrid lenses with other desired properties, such as hardness/machinability, wettability and other mechanical properties, polymer compositions containing hydrophobic silicone groups and hydrophilic groups were developed, the so-called silicone hydrogels. A variety of silicone hydrogel polymers have been disclosed as having high oxygen permeability, on-eye movement and tear exchange. For example, see U.S. Pat. No. 4,711,943 to Harvey and U.S. Pat. No. 5,260,000 to Nandu, et al.
While silicone hydrogels provide a better balance of the properties relevant to corneal health, their xerogel state before hydration suffers, in general, from poor machinability due to a low hardness. As a result, the preferred manufacturing mode primarily is using molding processes. Further, in the absence of suitable expansion control agents, silicone hydrogels suffer from the same mechanical shortcomings as traditional hydrogels when used in hybrid lenses. Furthermore, the expansion control agents used to control the expansion upon hydration in traditional hydrogels cannot be used in silicone hydrogels due to their substantial incompatibility to homogenously mix with the latter.
Thus, while many solutions have been explored to solve the problem of hybrid lens manufacture, none to date has proven satisfactory. Accordingly, there is a need for improved methods of manufacture which provide hybrid contact lenses that are easily machined and possess substantially little distortion when hydrated, while concurrently providing the mechanical and optical properties requisite for the function the hybrid lenses.